Guarded monopulse radar system

ABSTRACT

A Guarded Monopulse Radar System is disclosed for generating azimuth and elevation difference (or error) signals proportional to the angular displacement of the detected radar target from the antenna boresight axis and for generating a sum signal indicative of the target range. The azimuth and elevation difference signals are modulated in quadrature with sine and cosine signals of a reference frequency. The modulated signals are then added to a portion of the sum signal to provide a composite signal. The aforementioned steps are performed at microwave frequencies and the resultant signals are subsequently converted to IF signals by &#39;&#39;&#39;&#39;beating&#39;&#39;&#39;&#39; with a signal derived from a local oscillator. IF signals corresponding to the sum signal and the composite signal are amplified in a circuitry including fast, automatic gain control (AGC). The amplified signals are then demodulated in accordance with the reference signal to provide signals indicative of azimuth and elevation errors.

United States Patent Longuemare, Jr. et al.

GUARDED MONOPULSE RADAR SYSTEM Dec. 11, 1973 [5 7] ABSTRACT A GuardedMonopulse Radar System is disclosed for generating azimuth and elevationdifference (or error) [73] Assignee: Westinghouse Electric Corporation,signals proportional to the angular displacement of the Pittsburghdetected radar target from the antenna boresight axis [22] Filed; Feb18, 1972 and for generating a sum signal indicative of the target range.The azimuth and elevation difference s1gnals [21] PP 227,542 aremodulated in quadrature with sine and cosine signals of a referencefrequency. The modulated signals 52 u.s..c1... 343/16 M a en a d op p nof the sun signal to .pro- 51 Int. Cl. G015 9/22 vlde a Compost? slgnal-The aforemennoned Steps are [58] Field Of Search 343/16 M Performed atmlcrowave frequencles and thefesultam signals are subsequently convertedto IF signals by [56] References Cited beatingl with a signal derivedfrom a local oscillator. UNITED STATES PATENTS IF signa 5 correspondingto the sum slgnal and the composite signal are amplified 1n a circuitryincluding ghubb a1 m i fast, automatic gain control (AGC). The amplifiedsigeeren 3,710,390 1/1973 Krienheder 343/16 M nals are i ig q pif i h g;3,696,416 10/1972 Badiano et al. 343/16 M x creme to 8 sgna S came 0 andelevation errors.

Primary Examiner-T. l-l. Tubbesing Atz0rneyF. H. Henson et al. 8 Claims9 Drawmg F'gures *1 1 2 SlNum 1 1 SPIN 1 REFERENCE 1 1 GENERATOR (305 W1 1 1 1 HO r; 5 I260 I280 U) 1 w 8 1 "80 I200 FIRST MIXERS [220 IF AMPGATE g ggg 1 Z CHANNEL 1 TR 2- PAR AMP TORANGE 1 1 I52 TRACK FAST A607vEtoclnr I COMPOSITE Az 1 TR A PAR AMP 154 TRACK & l 151 an ANGLE "8b 3W1 ERROR SIGNAL CHANNEL IF AMP GATE AENVEL DETECWR L ..c ..sv v s..s I24DETECTOR ClRwlTS I32b 1ST LO SOURCE 13411 slNw t Dig AZ dc ERROR SIGNALE1 El 11.6 ERROR SIGNAL DEM.

COSw t PAIENTEBHEC II I975 3778329 sum 1 u; 4

, I. PRIOR ART XMTR I I BIIIEETIQNAL I I r COUPLER 078w [H60 fi I50 I063V: I I v AMPLITUDE MODULATOR I 48 I VAA VAA SlNUL I V E I42 I fflec3dbCOUPLER I smwu I46 2 OJOTEVAA SINULI +VAE COSUJ LI] I I I COSUL GEN.I g I I40b I g I VAE COSLO t I I I J O AMPLITUDE u ATOR 5& L 1 E} IOOMPOSITE A2 8 MODULATION CIRCUITRYEI EL ANGLE ERROR SIGNAL CHANNELZCHANNEL Y COMPOSITE MICROWAVE ANGLE ERROR CHANNEL WITH RE" INJECTEDCARRIER PATENTEDUEC I I I925 OUTPUT 3778329 SIIEEI II, III 4 FIG. 50

AA E AV SINaI g (4) C MICROWAVE CARRIER FREQ.

SINlJ t Az AMPLITUIE MO%- ULATG? 8I COUPLE ZCHANIIEL MICROWAVE COUPLEROUTPUT O VgSINw t FIG. 5d

0.45 v smw t DESIRED ENVELOPE O.45V: SINw t OSEAVASINAICIIIESINAJLCIFIG5e GUARDED MONOPULSE RADAR SYSTEM BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to radar systems and inparticular to guarded monopulse radar systems for producing azimuth andelevation signals indicative of the angular displacement of a radartarget from the antenna boresight axis.

2. Description of the Prior Art In the prior art, radar tracking systemssuch as beam lobing systems and monopulse systems are known. Thefundamental difference between these types of systems resides in theinstantaneous antenna pattern used and the signal processing techniques.In both of these systems, an error signal (or signals) is generated toindicate the angular displacement of the radar target from the antennaboresight axis. Such an error signal can then be used to controlsuitable servo drive systems to move the antenna in a direction to nullthe detected error. Typically, at least two signals are generated toprovide an indication of the detected error in terms of at least twoorthogonal coordinates in order to mechanize a practical,non-interacting servo system.

Typically, a monopulse radar system operates to pro vide error signalsindicative of the degree of angular displacement in azimuth and inelevation of the target from the reference direction. Such signals mayappear in the form of signal ratios which may be obtained by comparingpairs of received signals detected by respective pairs of antennaelements that are displaced in the horizontal and vertical planes, or anequivalent antenna array.

In comparing performance, a properly mechanized monopulse system has asignificant advantage over the various lobing systems, i.e. immunity toamplitude fluctuations of the radar signal emanating from the target.These signal fluctuations can be of natural origin such as thoseresulting from target scintillation or can result from intentionalamplitude modulation, e.g., angle track ECM jamming repeaters. Incontrast, lobing systems are intrinsically simpler and can provideexcellent angle tracking under many conditions with only a singlereceiving channel, whereas most monopulse systems require at least threeidentical channels. Such monopulse systems require accurate preservationof the relative phase angle of the signals transmitted through theplurality of channels, in the microwave portion and in at least a partof the IF portion of a typical radar receiver. This problem is greatlyaggravated in high performance doppler radars which utilize highlyselective filters to separate the desired target return signal fromother clutter or interfering signals. Typically, these filters introducelarge phase shifts which are usually difficult to match in each of themultiple channels of the system. Typically, such systems require meansfor automatic phase correction thus increasing the overall complexity ofthe system.

In a prior art monopulse system known as the Sum and Difference Systemand illustrated in FIG. 1, four signals are developed by two pairs ofhorizontally and vertically displaced antenna elements 12a, 12b, 12c andI211 of an antenna array 12 and are combined additively andsubtractively in a sum and difference network M to produce an azimuthdifference signal A an elevation difference signal A and a sum signal 2upon the output waveguides 16b, 16c and 16a, respectively.

Each of these three signals is a composite function of the azimuth andelevation displacement angles, and may be processed in a system shown inFIG. 2. A threepart circulator 17 is inserted within the waveguide 16afor coupling selectively a transmitter 15 to the antenna and Z/Acircuitry 10 during a transmit cycle. During the receive periodfollowing the transmitted pulse, the sum signal 2 and the differencesignals A, and A are typically passed through receiver protector tubes18a, 18b and 18 c and low-noise parametric amplifiers 20a, 20b, and 20cto establish a system noise figure. In a conventional manner, thesemocrowave signals are converted to intermediate frequency (IF) signalsby beating these signals in mixers 22a, 22b and 220 with signals derivedfrom a local oscillator 24. The resultant IF signals may be appliedthrough suitable IF amplifiers 26a, 26b and 260 to gates 28a, 28b and280, to respectively amplify and gate the signals. Up to this point, thethree channels as indicated by the letters a, b and c, are made asnearly identical as possible. The sum channel indicated by the letter acontains, once the target track is established, the full target signal,whereas the two difference channels indicated by the letters b and 0contain target signals A and A whose amplitude is a function of antennaangular pointing error. When the antenna is pointing directly at thetarget, no target signals exist in the A and A channels.

The signals developed at the gates 28b and 280 are applied to thesynchronous detectors 30a and 30b. As shown in FIG. 2, the signalderived from gate 28a is applied to the synchronous detectors 30a and30b and also to an envelope detector 32. The synchronous detectors 30aand 30b provide error signals respectively indicative of the azimuth andelevation errors. The envelope detector provides a signal 2 indicativeof the range or velocity of the target. Since the full target amplitudeappears in the sum channel, an AGC loop or circuit is connected as shownin FIG. 2. In particular, the AGC circuit from the envelope detector 32includes a summer 34 for comparing the output signal 2 with an AGCreference signal; the resultant signal which is applied to an amplifier36. The output of the amplifier 36 is applied to the IF amplifiers 26a,26b and 26c to control the gain thereof. By carefully designing andconstructing the two A channels to be nearly identical to the 2 channelas possible, the developed AGC signal can be used to vary the A channelsin a nearly identical manner. Thus, the proper sensitivity or scalefactor may be maintained on the azimuth and elevation error signals. Thedeveloped azimuth and elevation error signals may then be applied toservo drive systems associated with the antenna 12 to null the detectederror. In order to preserve the sense or direction information containedin the A channels, the A channels must be synchronously detected withthe 2 channel providing the necessary reference. Such a requirementrestricts the allowable phase difference between the channels and itaccounts for much of the complexity of a monopulse system.

In the prior art, monopulse radar systems have been suggested in whichthe number of channels required has been reduced. In US. Pat. No.3,339,l99, the number of channels in the receiver is reduced to a singlechannel. In particular, there is suggested an antenna array forgenerating four video signals which are applied to a sum and differencenetwork to provide a sum signal, an azimuth difference signal and anelevation difference signal. The signals so derived from the sum anddifference network are then converted to IF signals in a generallyconventional heterodyning step using mixers and a local oscillator.Subsequent to the IF conversion, the resultant IF azimuth and elevationsignals are modulated in quadrature. The modulated elevation and azimuthsignals are added at a suitable junction. A 90 phase shift is impartedto the sum signal which is then added with the sum of the error signalsto provide a composite sum signal. The composite signal is then passedthrough a limiting amplifier and a frequency discriminator; theresultant signal is then demodulated to provide the respective elevationand azimuth error signals.

Though the above-described system would have the advantage of reducingthe number of channels required, this system would not be readilycompatible with doppler radars. In particular, modulation andcombination of IF signals are performed after heterodyning andpreamplifying to permit splitting of the sum signal before the 90 phaseshift. This technique requires the maintaining of the phase relationshipbetween the various channels until at least after the error signals andsum signal have been added together. Further, the use of limiting,combined with a frequency discriminator, produces higher order sidebandsand harmonics of the modulating signal. Further, it is fundamental tomonopulse radar systems that amplitude variations in a target echo dueto scintillation or jamming modulation will effect the A and 2 channelsby the same proportionality factor. It would be desirable to avoid thesemodulation errors. In particular, doppler radar systems are particularlysusceptible to such errors and further are not adaptable to thetechniques suggested in the above-identified patent.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide a new and improved monopulse radar system in which the number ofchannels required is reduced.

It is a further object of this invention to provide a new and improvedmonopulse radar system compatible with doppler radar techniques.

It is a more particular object of this invention to provide a new andimproved monopulse radar system by reducing the phase variations betweenthe various channels and also by significantly reducing undesiredmodulation without incurring clutter spreading.

These and other objects are met in accordance with the teachings of thisinvention by providing a monopulse radar system comprising an antennaarray, a sum and difference network for providing an azimuth differencesignal, an elevation difference signal and a sum signal; a modulatingcircuit for modulating the azimuth difference and the elevationdifference signal; a summing circuit for combining the modulated signalswith a portion less than the whole of the sum signal to provide acomposite signal modulated in amplitude whereby the modulation componentincludes terms proportional to the azimuth and elevation angles;demodulating circuits for receiving the composite signal for providingseparate azimuth and elevation error signals; amplifying means forvariably amplifying the sum signal and the composite signal; and afeedback circuit responsive to the amplfied sum signal for variablycontrolling the gain of the amplifying circuits. As a result, theamplitude variation in the signals derived from the antenna aresubstantially eliminated and the azimuth and elevation error signals aresubstantially immune to target or externally generated signalmodulation.

In an illustrative embodiment of this invention, the modulating andsumming of the azimuth, elevation and sum channels is carried out atmicrowave frequencies to provide a composite signal. Thereafter, thecomposite signal and the sum signal are converted in IF signals byheterodyning techniques involving a mixer and a local oscillator source.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantagesof the present invention will become more apparent by referring I to thefollowing detailed description and the accompanying drawings, in which:

FIGS. 1 and 2 show diagramatically a monopulse radar system of the priorart;

FIG. 3 is a schematic representation of a portion of a monopulse radarsystem in accordance with the teachings of this invention;

FIG. 4 is a schematic representation of the monopulse radar system inaccordance with the teachings of this invention; and

FIGS. 5A, 5B, 5C, 5D and 5B are graphical representations of the signalsdeveloped within the circuits of FIGS. 3 and 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to thedrawings and in particular to FIG. 3, a front end assembly 116 is shownwhich is essentially similar to the assembly 10 shown in FIGS. 1 and 2.Illustratively, the front end assembly includes an antenna array withfour antenna elements or horns arranged in a similar manner to theelements of the antenna array 12 of FIG. I. The antenna array generatesfour video signals which are applied to a sum and difference network,similar to that shown in FIG. I, which illustratively includes fourhybrid junctions, each having a pair of inputs and outputs and acting todeliver at its output, the sum indifference of the two signals appliedto its inputs. In particular, the front end assembly 110 generates atits output terminals 1116a, 1161) and 1 16c a sum signal designated V 2a first difference signal designated V and a second difference signaldesignated V A respectively. The antenna array and the sum anddifference network used to obtain the signals and comprising the frontend assembly 110 are conventional circuits and thus have not beendescribed in detail. It may be further understood that the threecombination signals are similar to the basic signals of any sum anddifference monopulse receiver, i.e., the sum signal V2 is indicative ofthe target range or velocity; the difference signal V A A is indicativeof the azimuth error and the difference signal V A E is indicative ofthe elevation error. More specifically, each of the three signals V Z VA and V A A is a simultaneous function of the angular displacement ofthe target from the boresight (reference axis of the antenna array) inazimuth, and of the angular target displacement of the target inelevation.

A signal from. the radar transmitter 1-18 is fed through the 2 channelto the antenna array by a conventional circulator or duplexer 117. Thereceived sum signal V is fed through the circulator I17 and into adirectional coupler 150. The azimuth and elevation difference signals VV A however, are applied to amplitude modulators 1140a and 14Gb. A spinreference generator 142 proyides sine and cosine signals of a selectedlobing frequency (1),, to be applied to the modulators 140a and 14017,respectively, to modulate the azimuth and elevation signals V A and V AThe modulated azimuth signal (V A A sin 107 t) and the modulatedelevation signal (V A E cosine w t) are summed together by a coupler144. The unused terminal of the coupler 144 may be terminated in animpedance 146. Assuming a 3 dB coupler 144, the resultant signal derivedfrom the coupler 144 would be 0.707 [VAA sin w t V cosine mu].

Aportion of the sum signal V Z is derived from the 2 channel by thedirectional coupler 150. That signal derived from the coupler 150 has anamplitude less than the signal V 2 derived from the output terminal 116a(e.g., for the 4 dB coupler shown, the output is 0.63 of the input) andis combined with the modulated azimuth and elevation signals by acoupler 148 to provide a composite signal indicative of the azimuth andelevation angle errors. Thus, one channel of a monopulse radar systemthat would normally be required has been eliminated. The compositesignal may be represented as follows: 7 t

0.707 [0.63 V 2 +0.707 (V A A sin m t+ V A E cos FIGS. SA-SE illustratethe nature of the signals derived from the circuit shown in FIG. 3.Assuming an antenna azimuth pointing error AB an azimuth error signalVAA will be generated as shown in FIG. A upon the output terminal 11Gb.The azimuth error signal VAA may-be represented as:

VA A A VA Sin (D l, where w is the microwave carrier frequency. Asexplained above, the azimuth error signal V A A would be modulated witha lobing reference signal sin w t shown in FIG. 5B. In practice, thefrequency of a) would be much lower than that of w The amplitudemodulated azimuth signal is shown in FIG. 4C and may be represented asfollows:

0.5 [AV sin ta t] [sin w t] As noted in FIG. 5C, the amplitude modulatedazimuth signal is a double sideband, suppressed carrier signal with aphase reversal occurring when sin w t changes sign. FIG. 5D representsthat fraction of the 2 channel signal attained from the directionalcoupler 150 and attenuated by coupler 148 and may be represented by thefollowing:

0.45 V 2 sin m t.

The sum of this fractional 2 signal and the modulated azimuth signal isshown in FIG. 5E; the resulting signal is a conventional amplitudemodulated signal containing a'carrier and two sidebands. The effect ofadding the attenuated or fractional 2 channel signal V 2 was to reinsertthe carrier signal. The envelope of the signal shown in FIG. 5B is asinewave of frequency 0),, whose amplitude is directly proportional tothe azimuth angular error AB The modulation of this signal can be easilyextracted by merely envelope detecting this signal. A substantiallyidentical process is used for detecting the elevation angle error A0,;and except that the lobing modulation is applied in phase quadrature(i.e., the modulating signal is cos w t), thus permitting both theazimuth and elevation signals to be combined in a signal channel andseparated later by angle track demodulators.

With regard to FIG. 4, there is shown a schematic representation of theentire monopulse receiver system in accordance with the teachings ofthis invention. The modulation circuitry 151, as shown in FIG. 3, isincorporated into the circuit of FIG. 4 and is identified by the numeral151. By so combining the azimuth and elevation error signals to form acomposite error signal, only two channels are thereafter requiredinstead of three. As a result, control of the microwave and IF phaseshift between the two channels is no longer needed, since the desiredinformation is completely contained within the envelope of the compositesignal. As indicated in FIG. 4, the composite signal and the 2 signalare applied through receiver protector tubes l18 a an d 1185 to the lownoise parametric a'rnplifiers' a and 12012. The amplified signals areapplied with local oscillator signals derived from source 124, to mixers122a and 122b to provide corresponding IF signals. The resultant IFsignals are directed through variable gain IF amplifiers 126a and 12617to gates 128a and 128b, for successive amplification and targetselection gating.

The envelope of the composite signal derived from gate 128 is a sinewave of frequency m whose ampli tude and phase are directly related tothe azimuth and elevation angle errors. The composite signal is thenapplied to the envelope detector l32b to provide a signal correspondingto the envelope of the input signal. To separate the azimuth andelevation components of the composite signal, the composite signalderived from the envelope detector 132b is applied to the demodulatorcircuits 134a and 134b, to be respectively demodulated in accordancewith 'the sin wLt and cos wLt lobing reference signals. Since thedemodulators 134a and l34b reject the other quadrature phase signal, theresultant AZ and EL angular error signals are easily derived distinctfrom each other and may be applied to respective servo drives toreorient the antenna array to null the error signals.

In accordance with a significant aspect of this invention, the sumsignal 2 derived from the envelope detector 132a is applied to an AGCfeedback circuit to control the gain of the IF amplifiers 126a and12612. In particular, the sum signal 2 is applied to a summer 152, wherethe sum signal S. is compared to an AGC reference signal with theresultant output applied to and amplified by an amplifier 154. Theoutput signal derived from the amplifier 154 is applied to both the IFamplifiers 126a and 12612 to vary the gain of the amplifiers 126a and126b and therefore the amplitude of the sum and composite signalsdirected therethrotfgh.

In a monopulse antenna, amplitude variations in the target echo due toscintillation or jamming modulation will effect both the E and Achannels by the same proportionality factor. For example, an increae insignal strength of 10 dB will cause both the Z and A signals to increaseby a like factor of 10 dB. Thus, in accor' dance with the teachings ofthis invention, the AGC circuit senses these amplitude variations in the2 channel and varies the gain of the amplifiers 126a and 126b(associated with both the 2 and composite channels) in an inversemanner. If a fast" AGC circuit is used, i.e., an AGC circuit, whoseresponse time is small compared to a period of the lobing frequency (0the amplitude of the resulting 2 signal can be made substantiallyconstant over the range of amplitude modulation frequencies to which theangle error demodulators are sensitive. Since the composite channel isalso connected to the feedback circuit, the composite signal willsimilarly be independent of such target generated amplitudefluctuations. As a result, the AZ and El error signals derived from thesystem as shown in FIG. 4 are immune to target or externally generatedsignal modulation.

Thus, there has been shown a monopulse radar system employing twoinstead of three receiving channels. As a result, the critical phasebalancing is no longer required after the elevation and azimuth signalshave been combined at the microwave signal mixer. Further, by addingonly a portion of the sum signal to the azimuth and elevation signals,the splitting is carried out at microwave frequencies prior to anypreamplification, thus avoiding the unpredictable phase variationsassociated with low-noise parametric amplifiers and IF preamplifiers.Further, the monopulse radar system of this invention utilizes a fastAGC circuit to vary the gain of not only the 2 signal, but also of thecomposite signal to thereby eliminate the effect of amplitudescintillation noise or external ECM repeaters. In doppler radar systemsthis has a particular advantage, since the fast AGC circuit can beapplied subsequent to doppler filtering. Further, the radar system inaccordance with the teachings of this invention makes use of a puresinusoidal lobing modulator, as opposed to phase modulators, thusavoiding the higher frequency harmonic modulation and consequent clutterspreading associated with other types of monopulse systems which usemodulation techniques. Further, this system permits target trackingwithout excessive phase error in doppler radars when the targetfrequency is adjacent to very sharp clutter rejection filters whosephase match is difficult to control. Further, this radar system iscompatible with multimode radars using various combinations of high PRF,medium PRF, low PRF, pulse com pression and non-coherent pulse modes.The system in accordance with teachings of this invention eliminatesundesired conical scan modulation on the transmitter radar signal, thusremoving this type of modulation as a source of spurious signal when aradar is used for semi-active missile illumination. Further, this systemis fully compatible with rapid frequency diversity techniques since noRF phase matching or balancing is required between the parametricamplifiers and mixers of the system.

The sum channel output provides an error signal for range tracking,velocity tracking, and signal-presence detection which has no anglemodulation superimposed on it. In some antenna designs, the sum channelhas lower sidelobe levels than the difference channels, which allowsimproved range and velocity tracking and signal detection in thepresence of sidelobe clutter or jamming.

Numerous changes may be made in the abovedescribed apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit thereof; therefore, it is contended that all mattercontained in the foregoing description and shown in accompanyingdrawings, shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

I. In a monopulse radar receiver system comprising an antenna having anaxis, and network means coupled to the antenna for providing an azimuthdifference signal indicative of the azimuth error angle between the axisand the detected target, an elevation difference signal indicative ofthe elevation error angle between the axis and the detected target, anda sum signal, the improvement comprising:

reference means for providing first and second reference signals inquadrature with each iother; first and second modulators for receivingand modulating respectively the azimuth difference signal and theelevation difference signal in accordance with the first and secondreference signals; summing means for combining the modulated azimuth andelevation difference signals and a derived signal of n percent of thesum signal, where n percent is less than percent to produce a compositesignal whose amplitude modulation compo- V nents comprise termsindicative of the azimuth and elevation error angles;

first and second amplifier means each having a variable gain, forreceiving and variably amplifying the amplitude of the composite signaland the sum sig nal respectively;

demodulator means for receiving and demodulating the composite signal inaccordance with the first and second reference signals to provide firstand second error signals indicative respectively of the azimuth andelevation error angles; and

control circuit means responsive to the amplified sum signal to controlthe gain of said first and second amplifier means inversely as afunction of the amplitude of the amplified sum signal.

2. In a monopulse radar receiver system as claimed in claim 1, saidreference means is adapted to provide the first reference signal in theform of sine out and the second reference signal in the form of cos (ut, where an, is a selected reference frequency.

3. In a monopulse radar receiver system as claimed in claim 2, whereinsaid reference means is adapted such that 0),, is less than thefrequency w of the carrier signal of the radar system.

4. In a monopulse radar receiver system as claimed in claim 1, envelopedetector means for receiving the composite signal and for providing anoutput signal indicative of the envelope thereof.

5. In a monopulse radar receiver system as claimed in claim 4, secondenvelope detector means for receiving the sum signal and for providing asecond output signal indicative of the envelope thereof.

6. In a monopulse radar receiver system as claimed in claim 1, whereinthere is included coupler means associated with the network means forproviding the derived signal.

7. In a monopulse radar receiver system as claimed in claim 6, whereinsaid summing means includes a first coupler for combining the modulatedazimuth and elevation error signals to provide an intermediate signalindicative thereof and a second coupler for combining the intermediatesignal and the derived signal to provide the composite signal.

8. In a monopulse radar receiver system as claimed in claim 7, whereinthere is further included conversion means coupled to said secondcoupler for converting the composite signal from microwave tointermediate frequencies.

1. In a monopulse radar receiver system comprising an antenna having anaxis, and network means coupled to the antenna for providing an azimuthdifference signal indicative of the azimuth error angle between the axisand the detected target, an elevation difference signal indicative ofthe elevation error angle between the axis and the detected target, anda sum signal, the improvement comprising: reference means for providingfirst and second reference signals in quadrature with each other; firstand second modulators for receiving and modulating respectively theazimuth difference signal and the elevation difference signal inaccordance with the first and second reference signals; summing meansfor combining the modulated azimuth and elevation difference signals anda derived signal of n percent of the sum signal, where n percent is lessthan 100 percent to produce a composite signal whose amplitudemodulation components comprise terms indicative of the azimuth andelevation error angles; first and second amplifier means each having avariable gain, for receiving and variably amplifying the amplitude ofthe composite signal and the sum signal respectively; demodulator meansfor receiving and demodulating the composite signal in accordance withthe first and second reference signals to provide first and second errorsignals indicative respectively of the azimuth and elevation errorangles; and control circuit means responsive to the amplified sum signalto control the gain of said first and second amplifier means inverselyas a function of the amplitude of the amplified sum signal.
 2. In amonopulse radar receiver system as claimed in claim 1, said referencemeans is adapted to provide the first reference signal in the form ofsine omega Lt and the second reference signal in the form of cos omegaLt, where L is a selected reference frequency.
 3. In a monopulse radarreceiver system as claimed in claim 2, wherein said reference means isadapted such that omega L is less than the frequency omega c of thecarrier signal of the radar system.
 4. In a monopulse radar receiversystem as claimed in claim 1, envelope detector means for receiving thecomposite signal and for providing an output signal indicative of theenvelope thereof.
 5. In a monopulse radar receiver system as claimed inclaim 4, second envelope detector means for receiving the sum signal andfor providing a second output signal indicative of the envelope thereof.6. In a monopulse radar receiver system as claimed in claim 1, whereinthere is included coupler means associated with the network means forproviding the derived signal.
 7. In a monopulse radar receiver system asclaimed in claim 6, wherein said summing means includes a first couplerfor combining the modulated azimuth and elevation error signals toprovide an intermediate signal indicative thereof and a second couplerfor combining the intermediate signal and the derived signal to providethe composite signal.
 8. In a monopulse radar receiver system as claimedin claim 7, wherein there is further included conversion means coupledto said second coupler for converting the composite signal frommicrowave to intermediate frequencies.